U.S. patent number 5,065,759 [Application Number 07/575,289] was granted by the patent office on 1991-11-19 for pacemaker with optimized rate responsiveness and method of rate control.
This patent grant is currently assigned to Vitatron Medical B.V.. Invention is credited to Malcolm J. S. Begemann, Bernhard de Vries, Johannes S. van der Veen.
United States Patent |
5,065,759 |
Begemann , et al. |
November 19, 1991 |
Pacemaker with optimized rate responsiveness and method of rate
control
Abstract
A pacemaker system is provided for rate responsive pacing,
wherein rate is controlled as a function of two or more sensor
inputs, each sensor providing a signal representing a respective
different control parameter. Preferably a first sensor signal
represents a physiologically accurate although slow response signal
such as OT interval, and a second sensor represents a relatively
fast response such as activity. The two parameter signals are
processed so that they are directly comparable and can be compared
as indicators of pacing rate throughout the desired pacing range.
The algorithm utilizes a parameter control reference curve for each
respective parameter, such reference curve representing the desired
correlation between pacing rate and the parameter signal. Rate
control is accomplished by determining the difference between each
processed parameter signal and its corresponding reference point
for the current pacing interval, and logically analyzing the two
differences to determine which is used to indicate change in pacing
rate. Each parameter reference curve is automatically adjustable to
correspond to patient conditions. Automatic drift correction of the
fast response parameter, such as activity, is used to compensate
for conditions where the fast response signal is not likely to be
physiologically reflective of the patient condition.
Inventors: |
Begemann; Malcolm J. S. (Velp,
NL), de Vries; Bernhard (Dieren, NL), van
der Veen; Johannes S. (Arnhem, NL) |
Assignee: |
Vitatron Medical B.V. (Dieren,
NL)
|
Family
ID: |
24299690 |
Appl.
No.: |
07/575,289 |
Filed: |
August 30, 1990 |
Current U.S.
Class: |
607/18; 607/19;
607/25 |
Current CPC
Class: |
A61N
1/36585 (20130101) |
Current International
Class: |
A61N
1/365 (20060101); A61N 001/365 () |
Field of
Search: |
;128/419PG,707 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0222681 |
|
May 1987 |
|
EP |
|
2216011 |
|
Oct 1989 |
|
GB |
|
Primary Examiner: Kamm; William E.
Attorney, Agent or Firm: Woodcock Washburn Kurtz Mackiewicz
& Norris
Claims
What is claimed is:
1. A pacing system having rate controllable pulse generator means
for generating pacing pulses, first sensor means for developing
first signals indicating a first pacing rate, second sensor means
for developing second signals indicating a second pacing rate, and
rate control means for determining desired pacing rate and
controlling the rate of said pulse generator means, said rate means
further comprising algorithm means for determining (a) from each of
said first and second sensor means signals a respective indicated
direction of rate change and amount of rate change, and (b) for
determining said desired pacing rate as a function of said
directions and amounts.
2. The pacing system as described in claim 1, comprising a lead
having a first end with an electrode adapted for placement in the
patient's ventricle, the other end being electrically connected to
said pulse generator means, said first sensor means including said
lead and comprising means for detecting the QT interval, and
wherein said second sensor means provides an output indicative of
the patient's physical activity.
3. The pacing system as described in claim 1, wherein said first
signals indicate a physiological rate reflective of the
physiological state of said patient, and wherein said second
signals indicate a fast rate reflective of a relatively fast
response to patient conditions compared to said physiological
rate.
4. The pacing system as described in claim 1, wherein said second
sensor means has a relatively fast response to patient conditions
compared to the response of said first sensor means.
5. The pacing system as described in claim 3, wherein said
algorithm means comprises means for comparing said physiological
and fast rates, means for deriving a rate difference indication
therefrom, and means for determining pacing rate as the
physiological rate plus an increment of rate which is a function of
said rate difference.
6. The pacing system as described in claim 1, wherein said
algorithm means provides a signal for changing pacing rate by a
predetermined step when both said first and second sensor signals
indicate a rate change in the same direction.
7. The pacing system as described in claim 6, wherein said
algorithm determining means has means for comparing said amount
indications when said rate change directions are opposite, and
means for choosing the direction of rate change on the basis of
said amounts comparison.
8. The pacing system as described in claim 1, comprising means for
generating first reference data for correlating said first signals
and desired pacing rate and second reference data for correlating
said second signals and desired pacing rate, and wherein said
algorithm means comprises
means for comparing said first signals with said first reference
data to provide a first difference;
means for comparing said second signals with said reference data to
provide a second difference; and
means for comparing said first and second differences to determine
said desired pacing rate.
9. The pacing system as described in claim 1, comprising means for
automatically adjusting said second signals to indicate a pacing
rate closer to that indicated by said first signals.
10. The pacing system as described in claim 1, wherein said
algorithm means comprises means for comparing the respective
indicated directions of rate change and the indicated amounts of
rate change, and further comprises means for reducing the indicated
amount of change determined from said second signals relative to
said first signals.
11. The pacing system as described in claim 1, wherein each pacing
cycle said algorithm means selects which of said sensor signals
indicates a preferred rate change, and determines rate change in
accordance with the selected rate change.
12. A pacemaker system adapted for implantation in the body of a
patient, comprising:
pulse generator means adapted to be rate controlled for generating
pacing stimulus pulses;
at least first and second sensor means for generating respective
first and second pacing rate signals correlative of desired pacing
rates;
indicator means for deriving from each of said pacing rate signals
an indication for direction of desired change of pacing rate and an
indication of amount of desired change of pacing rate;
rate determining means for determining pacing rate as a
predetermined function of said direction and amount indications of
each respective sensor means; and
rate control means for controlling said pulse generator means in
accordance with said determined pacing rate.
13. A method of pacing utilizing a rate responsive pacing system
adapted to provide stimulus pulses to a patient's heart at a rate
within a predetermined range which is responsive to one or both of
a first physiological control parameter and a second fast response
control parameter, said system having means for periodically
obtaining signals reflective of said control parameters and
correlating means for correlating each parameter to a respective
indication of desired pacing rate, comprising:
a) correlating one of said parameter signals so as to adjust its
influence relative to the other control parameter signal, whereby
pacing rate is primarily determined by said first parameter signal;
and
b) comparing said respective pacing rate indications and
determining pacing rate as a function of said comparison each
pacing cycle.
14. The method as described in claim 13, wherein said comparing
step comprises establishing a parameter control reference point for
each respective parameter each time said control parameters are
obtained, each reference point representing pacing rate as a
function of the respective parameter signal.
15. The method as described in claim 14, comprising comparing for
each pacing cycle each control parameter with the reference point
established during the prior cycle, and establishing a difference
value therefrom, and said comparing step comprises comparing the
difference values so obtained for the two respective
parameters.
16. The method as described in claim 15, comprising automatically
introducing a correction factor to said second control parameter
whenever there is a difference between such control parameter and
its corresponding reference point.
17. The method as described in claim 14, comprising generating said
parameter control reference points in accordance with a
predetermined reference curve for each respective parameter, and
adjusting each said reference curve when a predetermined condition
exists at at least one pacing rate within said range.
18. The method of claim 13, comprising obtaining QT interval as
said first parameter and an activity count as said second
parameter.
19. The method of claim 13, comprising sensing patient heartbeat,
and obtaining both said first and second parameters from the sensed
heartbeat.
20. The method of claim 19, comprising obtaining QT interval as
said first parameter and T wave amplitude as said second
parameter.
21. A pacing system having pulse generator means for delivering
stimulus pulses and rate adjusting means for adjusting the rate of
delivery of said stimulus pulses, slow physiological sensor means
for generating a relatively slow response signal representative of
desired physiological pacing rate and at least a second fast sensor
means for generating a fast response signal having a faster
response than said physiological signal to changing patient
circumstances such as activity and the like and which is
representative of pacing rate, said pacing system being
characterized by:
slow means for deriving a physiological rate indication from said
slow signal;
fast means for deriving a fast rate indication from said fast
signal;
means for comparing said physiological and fast rate
indications;
means for determining a rate factor as a function of said
comparing;
means for determining desired pacing rate as said physiological
rate indication modified by said rate factor; and
means for causing said rate factor to move in a direction so as to
reduce its modifying effect.
22. The pacing system as described in claim 21 wherein said fast
sensor means comprises an activity sensor, and said slow sensor
means comprises means for obtaining QT interval.
23. The pacing system as described in claim 21, wherein both said
slow means and said fast means comprise means for obtaining patient
heartbeat signals and means for deriving signals representative of
desired pacing rate from said heartbeat signals.
24. A pacemaker system adapted to continuously vary the rate of
generated pacing pulses as a function of sensed patient conditions,
and having at least first and second sensors for obtaining signal
indications of pacing rate, said sensors having relatively high and
low respective rates of response to changing patient conditions,
the system having rate determining means characterized by:
reference means for determining a sensor rate reference for each of
said sensors corresponding to each pacing interval at which
stimulus pulses are generated;
difference means for determining a difference value for each sensor
for each pacing interval, each said difference value reflecting the
signal of each said sensor compared to the corresponding sensor
rate reference for the prior interval;
comparing rate means for comparing the difference values of each of
said sensors; and
rate means for determining rate change as a function of said
comparison of difference values.
25. The pacemaker system of claim 24, comprising means for coupling
said sensor rate references within a predetermined rate range.
26. The pacemaker system of claim 25, comprising means for setting
the maximum output of each of said sensor to be produced at the
upper limit of said rate range (URL) and at about maximum patient
exercise and stress.
27. The pacemaker system of claim 24, comprising means of adjusting
the relative influence of the signals of said sensors in
determining rate change.
28. The pacemaker system of claim 24, wherein said rate means
comprises step means for determining the step size of said rate
change, said step size being variable with pacing interval.
29. The pacemaker system of claim 24, comprising means for
periodically adjusting the fast rate response signal when at least
a predetermined difference value exists between said fast signal
and its corresponding reference value.
30. The pacemaker system of claim 24, wherein said reference means
comprises adjustment means operative under predetermined patient
conditions for adjusting the determined references for at least one
of said sensors.
31. A pacemaker system adapted to continuously vary the rate of
generated pacing pulses as a function of sensed patient conditions,
having rate means for controlling pacing rate over a predetermined
range of rates in response to a determination of rate change, said
means for controlling comprising:
first means for developing a first indicator of pacing rate based
on a first sensed parameter;
second means for developing a second indicator of pacing rate based
on a second sensed parameter which is a relatively faster-response
parameter compared to said first parameter;
said second means having conversion means for converting said
second indicator so that it has a proportionately lesser influence
relative to said first indicator; and
algorithm means for comparing said first indicator and said
converted second indicator and for determining change in pacing
rate as function of said comparing.
32. The pacemaker system as described in claim 31, wherein said
algorithm means is operative each pacing cycle and further
comprises means for determining the amount of rate change indicated
by each of said indicators.
33. The pacemaker system as described in claim 31, wherein said
first means comprises means for developing a signal representative
of a QT time interval, and said second means comprises means for
developing a signal representative of patient activity, and wherein
said conversion means converts said activity signals so that said
second indicator is representative of a time interval comparable to
said QT interval.
34. The pacemaker system as described in claim 31, wherein said
algorithm means comprises means for comparing the magnitudes of
said first indicator and said converted second indicator, and
comprises logic means for determining the direction of rate
increase as a function of said compared magnitudes when said
indicators indicate respective different directions of rate
change.
35. The pacemaker system as described in claim 31, wherein said
algorithm means performs said comparing periodically, and comprises
means for incrementing or decrementing rate by a selected
predetermined amount.
36. The pacemaker system as described in claim 31, wherein said
first parameter provides a relatively slow physiological response
to patient conditions and said second parameter provides a
relatively fast response to patient conditions.
37. The pacemaker system as described in claim 36, wherein said
first parameter is QT interval, and said second parameter is
activity.
38. The pacemaker system as described in claim 36, wherein said
second parameter is derived from the patient T wave.
39. The pacing system as described in claim 31, wherein said
conversion means converts said second indicator to have a lesser
influence relative to said first indicator throughout said
predetermined range.
40. The pacing system as described in claim 31, wherein said
conversion means performs the function of converting said second
rate indicator generally to a lower rate than the rate indicated by
said first rate indicator.
41. A pacemaker having pulse generator means for delivering
stimulus pulses and rate adjusting means for adjusting the rate of
delivery of said pulses, first sensor means for generating a first
response signal representative of desired pacing rate and at least
a second sensor means for generating a second response signal
representative of the desired pacing rate, said pacemaker being
characterized by:
a) first rate means for deriving a first rate change indication
from said first response signal; second rate means for deriving a
second rate change indication from said second response signal;
and
b) algorithm means operative each pacing cycle for comparing said
rate change indications and for selecting one of said rate change
indications, and rate means for incrementing or decrementing rate
as a predetermined function of said selected rate indication.
42. The pacemaker as described in claim 41, wherein said algorithm
means is further characterized by having means for automatically
decreasing rate changes indicated by said first response
signal.
43. The pacing system as described in claim 41, wherein one of said
rate means comprises influence means for adjusting its rate change
indication relative to the other rate change indication, whereby
one of said response signals normally has greater influence than
the other on determining rate changes.
44. A pacing system having pulse generator means for delivering
stimulus pulses and rate adjusting means for adjusting the rate of
delivery of said stimulus pulses, first sensor means for generating
a first signal representative of a first desired pacing rate and at
least a second sensor means for generating a second signal
representative of a second pacing rate, said pacing system being
characterized by:
first rate indication means for deriving a first rate indication
from said first signal;
second rate indication means for deriving a second rate indication
from said second signal;
said rate adjusting means having comparing means for comparing said
first rate indication and said second rate indication to provide a
rate control signal for control of said rate adjusting; and
drift means for changing said second rate indication in a direction
to reduce its influence relative to said first rate indication in
said comparison.
45. The pacing system as described in claim 44, wherein said first
sensor means generates a relatively slow response signal
representative of a physiological pacing rate and said second
sensor means generates a fast response signal having a faster
response than said physiological signal.
46. The pacing system as described in claim 45, wherein said drift
means changes said second rate indication by a fixed amount once
every predetermined time period.
47. The pacing system as described in claim 46, wherein said drift
means changes only a second rate indication which is false positive
compared to said first rate indication.
48. The pacing system as described in claim 46, wherein said drift
means corrects for second rate indications which are false
negatives compared to said first rate indications.
49. A pacemaker system having means for providing stimulus pulses
to a patient's heart at a rate which is responsive to one or both
of a first physiological parameter and a second relatively
faster-response parameter, said system having sensor means for
obtaining signals reflective of said parameters and correlation
means for correlating each of said parameters with a respective
correlation function to respective first and second indications of
pacing rate, said correlation means further comprising influence
means for setting the influence of said parameter signals relative
to each other so that pacing rate is primarily determined by said
physiological signal, and having comparing means for comparing said
first and second pacing rate indications and means for determining
pacing rate as a function of said comparison.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The subject invention relates to cardiac pacemaker systems and,
more particularly, implantable cardiac pacemakers which deliver
pacing stimulus pulses at an adjustable rate based upon monitoring
of patient conditions.
2. Description of the Background and Prior Art
Rate responsive pacemaker systems are widely available in the art.
Rate responsive systems contain means for monitoring at least one
patient variable and for determining an indicated pacing rate as a
function of such sensed pacing variable, so as to control pacing
rate optimally in terms of the patient condition. Such rate
responsive pacemakers have gained wide acceptance as providing an
improved response to the patient's physiological needs, as compared
to programmable fixed rate pacemakers. Although atrial-based
pacemakers, i.e. atrial synchronous or atrial sequential
pacemakers, as well as DDD pacemakers, may in some patients provide
an ideal form of rate responsiveness, such pacemakers are not
satisfactory for many patients with cardiac conditions.
A number of patient variables or rate control parameters have been
suggested in the technical literature and used commercially. One of
the first physiological parameters utilized for rate control is the
QT interval, as disclosed in the U.S. Pat. No. 4,228,803 to
Rickards, and the U.S. Pat. No. 4,305,396 to Wittkampf et al. The
QT interval is in fact the interval between a delivered pacing
stimulus and the subsequent evoked T-wave, and has been utilized as
the parameter indicative of physiological demand for heart output,
and thus pacing rate. Additionally, activity sensors have been
widely utilized for detecting the general activity level of a
patient with a pacemaker, and for controlling the pacing rate or
escape interval in response to detected activity level. See the
U.S. Pat. No. 4,428,378 to Anderson et al. Other parameters which
have been utilized or investigated for suitability as controlling
pacing rate include respiration rate, thoracic impedance changes,
venous blood temperature, pH, oxygen saturation and stroke
volume.
In addition to the selection of a desired monitored parameter, and
the corresponding sensor to be used, the algorithm utilized by a
pacing system is of great importance. An example of an improved
rate adaptive algorithm used in a pacing system is set forth in
U.S. Pat. No. 4,972,834, Ser. No. 252,653, filed Sept. 30, which
discloses a QT pacemaker with dynamic rate responsiveness,
incorporated herein by reference. As set forth in this referenced
patent, the algorithm which correlates the monitored or sensed
parameter with indicated pacing rate may be adapted as a function
of history, and particularly can be readjusted with respect to
limits such as lower rate limit (LRL) and upper rate limit
(URL).
Another approach to optimizing rate responsiveness is to use dual
or plural sensors, in order that the drawbacks or deficiencies of a
given sensor and/or algorithm may be compensated by the use of a
second or other sensors having different characteristics. This
approach is set forth in the patent to Rickards, U.S. Pat. No.
4,527,568, which discloses switching control of rate responsiveness
from one monitored parameter, e.g. atrial rate, to another control
parameter, e.g. QT interval. There are many other examples of dual
sensor approaches in the literature, and reference is made to U.S.
Pat. Nos. 4,926,863 and 4,905,697. These references are
characterized by designs which switch control from one sensor to
another, or from one algorithm to another, depending upon monitored
values of the rate control parameters. While this approach may
produce increased efficiency and improvement over the single sensor
approach, it still does not provide a continuous optimization of
information such as is potentially available from two or more
sensors, so as to continuously optimize and adapt the actual pacing
rate for all foreseeable conditions. As used in this specification,
"sensor" or "sensor means" refers to any means for obtaining a
control parameter, including the lead means such as is used for
obtaining the QT interval, or other sensors such as in use for
detecting body activity and the like. The techniques for sensing
rate control parameters, and developing and processing therefrom
signals useful for pacemaker control, are well known in the
art.
A longstanding unsolved problem in this art area, for which there
is a need for improvement, is thus to provide either a sensor or
combination of sensors which more nearly fulfills the requirement
of the ideal rate adaptive system. For example, a rate adaptive
system should provide a quick and accurate initial response to
situations such as start of exercise. The QT interval, as a rate
control parameter, provides only a gradual response, as compared to
an activity sensor which provides a fast, i.e., quick response.
Another requirement is that the parameter or parameters chosen
should provide an indication of pacing rate which is proportional
to the work load. The QT interval provides a very good indication
of work load, whereas the activity sensor approach is not as good,
and may be subject to false indications. Another important
requirement for a rate control parameter is specificity, i.e., that
the characteristics of the parameter signal are specific to the
conditions of rest and exercise of the patient and are thus
physiologically appropriate. For example, the QT interval has a
high specificity, whereas activity as a parameter has a medium
specificity. Yet another requirement is providing an optimum
indication of rate decrease following cessation or reduction of the
condition compelling higher rate, such as exercise. It is important
that the speed of rate decay after the cessation of exercise be
properly related to the patient's physical condition. It is known
that a patient in relatively poor physical condition experiences a
slow decrease of the heart rate after exercise, while a person in
relatively good physical condition experiences a more rapid
decrease of the heart rate after exercise. A pacemaker controlled
by an activity sensor is less than optimal in this regard, since a
cessation of exercise results in a sharp drop in the activity
signal which, if not modified, would lead to a non-physiological
step-like reduction in pacing rate. As a consequence, it is
necessary to program a fixed time period for gradually decreasing
the pacing rate when the activity sensor stops delivering
information calling for a higher rate. The pacemaker which is
controlled by the QT interval exhibits the inverse relationship as
known from exercise physiology, but tends to provide too slow a
pacing rate decay.
What is thus sought in this art area is a pacer having two or more
sensors and an algorithm for deriving information from each so as
to optimize the determination of desired pacing rate. At the start
of exercise, for example, it is desired to have the algorithm force
an initial but limited fast rate increase. Thereafter, it becomes
important to ensure that the pacing rate correlates proportionally
to work load, and that if continuous exercise is not confirmed, the
pacing rate will slowly decrease toward a lower limit. The
algorithm, combined with the sensing means, should also force a
faster, although limited rate decrease when stop of exercise is
detected, with further rate decrease following the physiologically
inverse relationship.
As is well known, the microprocessor and logic circuit technology
for dealing with these problems in a pacemaker environment is
available. What is needed is a pacemaker system which utilizes this
technology so as to optimize the translation of plural sensor
information into pacing rate control.
SUMMARY OF THE INVENTION
The pacing system of this invention provides an improvement in rate
responsive pacemakers so as to more optimally adapt rate control to
patient conditions. Specifically, the object is to combine
information from two or more sensor sources so that during all
phases of the patient's activity and rest there is provided
information accurately reflective of the physiological state of the
patient, as well as fast response information from which more
appropriate time control of pacing rate can be derived and
accomplished.
A specific object of this invention is to provide a pacing system
which is rate responsive to a quick response sensor which monitors
a parameter such as activity, as well as a slower response sensor
which monitors a more specific parameter such as QT interval,
having an algorithm and control means for detecting which sensor
source provides the optimum information under any given patient
condition, and controlling pacing rate in response to the detected
optimum information so as to provide physiologically optimal
pacing.
It is a further object of this invention to provide rate control
pacing where the pacing rate is primarily controlled in response to
first sensor information which is highly specific to patient
physiological conditions and provides an indication proportional to
work load, and wherein the controlled pacing rate is modified
according to further information from one or more other sensor
sources which provide a quicker response to patient conditions,
than does the first sensor source.
It is yet a further object of this invention to provide an
inplantable pacemaker which is rate controlled as a function of at
least two parameters reflective of patient condition, the pacemaker
utilizing an algorithm adapted to provide primary control on the
basis of a first of said parameters, and having means for adjusting
and coupling the control information derived from the second
parameter so as to be adapted for use in the same algorithm,
thereby giving the pacemaker the advantage of continual comparison
of comparable information from at least two parameter sources for
logically deciding what pacing rate is appropriate.
In accordance with the above objectives, there is provided a
pacemaker system and method of controlling pacing rate wherein at
least two rate control parameters are utilized. The system
incorporates means for processing a second one of the parameter
signals so that the respective parameter signals are comparable,
i.e. can be compared by a logic analysis as part of an algorithm in
determining how to control pacing rate. This system includes
establishing parameter control reference curves for each respective
parameter, each reference curve representing pacing rate (or pacing
interval) as a function of the respective parameter signal, and
each of the two or more parameter curves being coupled so that each
parameter indication is logic comparable over a range of pacing
rates. Primary rate control is based on a first parameter such as
QT interval, and rate control is modified by information from a
secondary source such as an activity sensor. Each reference
parameter curve is automatically adjustable to correspond to
patient conditions and patient history, while maintaining the
curves coupled. Automatic drift correction of the secondary
parameter, such as activity, is used where there is a difference
between the variable and its corresponding reference curve point
whereby one can choose to correct only positive differences, only
negative differences or both, so as to maintain comparability of
the variables, permitting continuous interval-to-interval comparing
and decision making on rate control.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic block diagram of a pacing system with
microprocessor control, such as can be used for this invention.
FIG. 2 is a generalized flow diagram showing some of the main
features of the system and method of this invention.
FIG. 3 shows a set of curves representing QT as a function of
pacing interval for different patient stress levels.
FIG. 4 shows the QT reference curve (QT.sub.ref), which is a
function of pacing interval, in relation to curves of QT for rest
and maximum exercise.
FIG. 5 shows a curve of ACT.sub.ref as a function of interval in
relation to curves of ACT for different exercise levels.
FIGS. 6A and 6B show coupled curves of QT.sub.ref and ACT.sub.ref
as a function of pacing interval, as used in the combination sensor
algorithm of this invention.
FIGS. 7A and 7B illustrate graphically, with respect to the coupled
reference curves, a first situation where both sensor variables
indicate an increased rate.
FIGS. 8A and 8B illustrate graphically a second situation where
both sensor variables indicate a decreased rate.
FIGS. 9A and 9B illustrate graphically a third situation where the
algorithm indicates an increased rate.
FIGS. 10A and 10B illustrate graphically a fourth situation where
the algorithm indicates a decreased rate.
FIGS. 11A and 11B illustrate graphically a fifth situation where
the algorithm indicates a decreased rate.
FIGS. 12A and 12B illustrate graphically a sixth situation where
the algorithm indicates an increased rate.
FIGS. 13A and 13B illustrate graphically a situation where the ACT
variable has a lesser influence than the QT variable.
FIGS. 14A and 14B illustrate graphically a situation where ACT and
QT have about the same influence.
FIGS 15A and 15B illustrate graphically a situation where ACT has a
greater influence than QT.
FIG. 16 is a flow diagram of the portion of the preferred algorithm
of this invention for changing pacing rate as a function of sensed
QT and activity signals.
FIG. 17 is a flow diagram of the portion of the preferred algorithm
of this invention for adjusting operation of the pacer at URL, and
of a portion of the algorithm of the preferred embodiment of this
invention for setting the activity drift (ACT.sub.dr) factor under
predetermined conditions.
FIGS. 18A and 18B illustrate graphically a situation where the
algorithm indicates start of ACT drift, causing ACT to
decrease.
FIGS. 19A and 19B illustrate graphically the situation sometime
following that of FIGS. 18A and 18B, where rate has decreased and
ACT drift has increased to the stable condition where
ACT=ACT.sub.ref and QT=QT.sub.ref.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In discussing the pacemaker system of this invention, reference is
made to the prior art which teaches the use of microprocessor
capability in an implantable pacemaker system, as well as the art
of external programmer communication with an implanted pacemaker.
Reference is made to U.S. Pat. Nos. 4,527,568 and 4,503,857,
incorporated herein by reference, which describe operations of
pacemaker embodiments incorporating microprocessor logic and
software algorithms. U.S. Pat. Nos. 4,228,803 and 4,305,396
describe the operation of embodiments of a Tx pacemaker, i.e., one
which is rate responsive to the QT interval, and are incorporated
by reference. Generally, the prior art teaches and discloses
various means of using microprocessors and software in controlling
the operation of an implanted pacemaker. Accordingly, the
specification does not contain a detailed description of the
commercially available and known techniques of programming a
microprocessor, storing data in memory and retrieving it, carrying
out such operations as timing time intervals and setting up sensing
windows, the logic of resetting the escape interval when a natural
heartbeat is sensed, etc. These operations are well known in the
art and are taught by the above references as well as other
published patents and articles in this area. Reference is also made
to the commercially available ACTIVITRAX pacemaker, of Medtronic,
Inc. and the QUINTECH pacemaker of Vitatron Medical B.V., as
commercial embodiments of software controlled pacers which are rate
responsive to activity and QT interval respectively, and to the
literature describing these pacemakers.
The following Glossary defines terms which are referred to in the
description of the preferred embodiments:
GLOSSARY OF TERMS
ACT: Control parameter representing activity
ACT.sub.ref : Reference value of activity control parameter at a
given interval (pacing rate)
ACT.sub.dif : ACT.sub.ref -ACT
ACTdr: Drift factor used to adjust
ACTmpl: Conversion factor by which activity counts (Nact) is
multiplied to convert to units of ms
B: second order constant in polynomial relating QT.sub.ref to
interval; primary factor in determining value of QT.sub.ref at
URL
Dact: value of incremental change in ACT.sub.ref for decremental
change in interval
Dpt: first order in polynomial relating to QT.sub.ref and interval;
value of incremental change in QT.sub.ref for incremental change in
interval
Fact: Activity frequency for activity sensor
Int: Pacing interval in ms
LRL: Lower rate limit of pacing range
N: Number of intervals or number of 3.3 second periods counted in
determining Nact
Nact: Number of counts of activity sensor
QT: Interval between delivered pacing pulse and evoked T wave; also
referred to as QT interval.
QT.sub.ref : Value of QT reference curve corresponding to a given
interval (Ttx).
QT.sub.dif : QT.sub.ref -QT
QT.sub.STR(max) : Maximum difference in QT due to increase in
stress, i.e., difference in QT at a given interval as between
patient at rest and patient at maximum exercise.
QT.sub.max : Lowest value of QT for a patient at maximum exercise
and URL
QT(m): Maximum value of QT at LRL and patient at rest
QTsave: QT.sub.ref -CRVMAX
Ttx: Interval on QT.sub.ref and ACT.sub.ref curves corresponding to
pacing interval
TwA: Amplitude of T wave
T.sub.URL : Interval at upper rate limit
T.sub.LRL : Interval at lower rate limit
URL: Upper rate limit of pacing range
LOOP: Software number corresponding to step size of increase in
pacing interval
CRVMAX: Programmable value representing threshold change in QT at
URL for determining whether QT.sub.ref should be changed at URL
In the subject invention, the improvement lies primarily in an
algorithm for processing data from two or more sensor sources, so
as to optimize rate control. The algorithm is structured so as to
go through routines each pacing cycle, making rate change decisions
based on a comparison of indications from each of the sensors. In
the preferred embodiment as illustrated, pacing rate is adjusted
each cycle, or interval, but the invention is not so limited. Thus,
the principles of the invention can likewise be applied to
adjusting pacing rate every N cycles, every elapsed predetermined
time period, etc. Also, while the preferred embodiment illustrates
a two sensor mode where the pacemaker is responsive to QT and
activity information, it is to be understood that other
combinations of two or more sensor inputs may be utilized. As used
herein, sensor, or parameter sensor, broadly refers to obtaining
rate control information and processing for use as control signals,
and embraces systems such as the QT system where the pacing lead is
utilized, as well as other systems where two or more separate
sensors are utilized.
Referring to FIG. 1, there is shown a schematic representation of a
pacer system as utilized in this invention. A pacemaker, preferably
an implantable pacemaker 50, is used with an external programmer
51, the external programmer operating in a known manner to program
pacemaker variables. A lead 53 is illustrated as being a
ventricular lead, which lead delivers stimulus pulses to the
ventricle and also provides sensed patient heartbeat signals to the
pacemaker in a known manner. A lead 54 may also be used for a dual
chamber pacemaker, connecting the pacemaker to the atrium. Leads 55
and 56 are shown connected to sensors S1 and S2 in the patient, for
providing control parameters as discussed above. For a system using
the QT and activity parameters, only leads 53 and 55 would be
required. The pacemaker is illustrated as having a microprocessor
57, which includes memory for carrying the software of the
algorithm of this invention. A block 58 is also indicated for
signal processing of signals derived from lead 53 and sensors S1
and S2 in the known manner. The pacemaker contains the conventional
means for generating stimulus pulses, inhibiting on demand,
controlling rate, pulse width, etc.
By way of overview, the algorithm of the preferred embodiment of
this invention is based upon several principles, or premises. A
first working premise is that one of the parameters is taken as the
primary control parameter, e.g. QT, and the other parameter is
converted into corresponding units so as to be comparable for
control purposes. Thus, in the preferred embodiment the ACT
variable is derived by first obtaining counts from an activity
sensor. See U.S. Pat. No. 4,428,378, incorporated herein by
reference. The count is converted into a variable having units of
ms, the same as QT, and is adjusted by a multiplier factor so as to
provide a comparison of the two variables throughout the
anticipated pacing range between lower rate limit (LRL) and upper
rate limit (URL).
Another principle of the algorithm of this invention is that a
reference curve is established for each variable, the reference
curve establishing desired correlation of the control variable and
the pacing interval. Actual control of pacing is preferably
accomplished on the basis of the determined difference between the
value of each respective control parameter at a given interval
compared to the corresponding reference value of that parameter at
the point on the reference curve corresponding to the pacing
interval. Thus, for the QT parameter, the algorithm operates each
interval to determine the difference between the QT reference value
(QT.sub.ref) for the pacing interval and the measured QT, to get
QT.sub.dif. Likewise, the algorithm makes a determination of the
difference between the activity reference (ACT.sub.ref) and the
determined ACT parameter, to obtain a value of ACT.sub.dif. Change
of pacing rate, which in the preferred embodiment is accomplished
each interval or pacing cycle, is determined as a function of these
difference values, and not as a function of a comparison of the
variable for the current interval versus the variable for the prior
interval. Each interval, or cycle, the reference value for each
pacing parameter is updated, or adjusted. Thus, for each new
interval, the corresponding reference value is either incremented
or decremented in accordance with a predetermined formula which
establishes the reference curve for each respective parameter.
The reference curves are coupled in the sense that each reference
curve represents a predetermined relation, or correlation, between
pacing interval and its respective parameter between LRL and URL.
In a preferred embodiment, the reference curve for the primary
parameter, QT, is established in accordance with measured patient
response to stress, and the second reference curve is scaled with
respect to the first reference curve in a manner such as to adjust
the influence of each parameter in determining control of pacing
rate. Thus, the invention embraces the use of two or more control
parameters, and an algorithm for establishing a respective
reference curve for each such parameter. The reference curves are
coupled so as to represent respective predetermined relationships
between pacing rate and the control parameters between the
programmable rate limits of the pacemaker, the algorithm
determining change in pacing rate as a function of determined
differences between each parameter and its respective reference
value at the current pacing interval in accordance with
predetermined logic.
Referring now to FIG. 2, there is shown a generalized flow diagram
which represents the basic steps of the algorithm of this invention
for determining pacing rate as a combined function of two control
parameters derived from separate sensor sources. An external
programmer 60 communicates with the pacemaker device. In general,
the programmer is used for programing different pacing operating
constants as is conventional in the use of programmable pacemakers.
In this case, the programmer provides data which is used for
generating the coupled reference curves, as indicated in block 65.
A fast non-physiological sensor, such as an activity sensor, is
indicated at block 61, and provides a signal which is operated on
to get a fast sensor rate indication, as indicated at block 63.
Note that the step of obtaining the fast sensor rate utilizes
reference curve information, i.e., the sensor indication is
compared to the reference information and the difference is used to
determine the fast sensor rate. Likewise, a slow physiological
sensor, such as means for obtaining the QT interval, is indicated
at 62. Its output is processed at 64 to get a slow sensor rate,
utilizing the correlation information generated at block 65. The
fast sensor rate and the slow sensor rate are compared at 66, and
the rate difference is adjusted at block 67. This adjustment
constitutes changing the difference by a drift factor to get a
differential adjustment indicated as .DELTA.R. At block 68, the
pacing rate is determined as the physiological rate (the slow
sensor rate) plus an increment determined by multiplying .DELTA.R
by a factor C. Thus, the pacing rate is determined as a primary
function of the physiological, or slow rate, and adapted as a
function of the fast sensor rate, the adaptation being adjusted by
the drift factor. It is to be understood that the adjustment of the
contribution of the secondary sensor may be obtained by different
procedures other than the drift factor as used herein, e.g., by
differentiation of the rate difference or another mathematical
operation.
As used herein, drift refers to a periodic change in the
contribution of the fast, or secondary sensor rate. For example,
where the fast sensor indicates a rate higher than the
physiological rate, such that the value of .DELTA.R is positive,
the pacemaker may decrement the positive value of .DELTA.R once
every predetermined number of seconds, causing .DELTA.R to drift
toward zero and thus reduce over a period of time the differential
effect of the fast sensor. Likewise, where the .DELTA. rate is
negative, meaning that the pacing rate is below the physiological
rate due to the contribution of the fast sensor rate, the drift
factor would increment .DELTA. rate so that the negative .DELTA.
rate drifted toward zero with time. The effect of introducing such
a drift factor is thus to initially permit the influence of the
fast sensor on the premise that its fast response is more accurate
than that of the slow sensor in reacting to changes, but to
diminish with time the differential influence of the fast sensor on
the grounds that after a while the slow sensor is more reliable. By
only allowing a decremental drift of .DELTA. R or only an
incremental drift of .DELTA.R one can choose to only diminish rates
which are higher or lower than the physiological rate.
It is known that the QT interval is in fact influenced both by the
pacing interval and by mental and physical stress. Thus, when the
pacing interval shortens, QT shortens likewise, and vice versa, in
a non-linear manner. Both mental and physical stress cause QT to
shorten, and for this invention it is assumed that the relation
between stress change and QT change is substantially linear. FIG. 3
shows curves for this relationship corresponding to the patient at
rest (1), medium exercise (2) and maximum exercise (3).
QT.sub.STR(max) indicates the change in QT at URL between rest and
maximum exercise. The QT interval is obtained in the ordinary
manner, with circuitry and/or software for timing out the interval
between the delivered stimulus pulse and the sensed T wave. See,
for example, U.S. Pat. No. 4,527,568.
The QT.sub.ref curve, which correlates QT as a function of
interval, is illustrated in FIG. 4. The QT.sub.ref curve must be
maximum at LRL and zero stress (patient in rest condition) and must
be minimum at URL and maximum stress (maximum patient exercise).
The QT.sub.ref curve between these two limits is chosen such that
the relation between QT.sub.ref and interval is a second order
polynomial function, providing that the step change in QT.sub.ref
is a linear function of interval:
FIG. 4 illustrates the determination of QT.sub.dif =(QT.sub.ref
-QT) at a given interval. Thus, if at the pacing interval
designated I.sub.o, the actual QT is less than the reference curve,
as indicated at point 3i, then QT.sub.dif is greater than 0,
indicating a desired increment in the pacing rate (decreasing
pacing interval). On the other hand, if the measured QT is greater
than the reference value, then QT.sub.dif is negative, indicating a
decrease in pacing rate (increase in interval). This is summarized
as follows:
______________________________________ Point 3i: QT < QT.sub.ref
QT.sub.dif = (QT.sub.ref - QT) > 0 increment pacing rate Point
3d: QT > QT.sub.ref QT.sub.dif = (QT.sub.ref - QT) < 0
decrement pacing rate ______________________________________
To obtain the second pacing parameter (ACT) a piezo-electric
element (which may be S1 in FIG. 1) is suitably glued inside the
pacemaker can. The activity sensor delivers an electrical signal
that depends on vibrations of the can, which in turn are caused by
movements of the patient and thus relate to exercise. The
electrical signal is sensed by a sense amplifier, and the number of
activity senses is counted. This number, Nact, is assumed to be
proportional to the physical stress, and thus a measure thereof.
Reference is made to U.S. Pat. No. 4,428,378, assigned to
Medtronic, Inc., which discloses a rate adaptive pacer utilizing an
activity sensor, and which is incorporated herein by reference.
The activity signal Nact, which is a unitless number, is converted
in the practice of this invention to have the same units as QT,
i.e. ms, by the following formula:
By this conversion, Nact can be compared to the QT variable in a
combined QT plus ACT algorithm. It is noted that the number of
activity senses can be counted in two ways. In a first method, a
programmable time is utilized, providing the following formula:
Alternately, Nact can be a counted over predetermined number (N) of
intervals, resulting in the following equations:
In a preferred embodiment, means for obtaining the ACT signal are
controlled by a software routine which is entered once every cycle.
If the system is programmed to utilize the activity signal, the
routine checks to see if Nact is to be read on an interval or time
basis. If the moment to read has not arrived, the routine skips
out. However, if the moment to read is present, the routine gets
the counts (Nact) from the activity sensor and resets the activity
counter for the next cycle. Then, optionally, the routine may limit
any change in Nact compared to the prior value, if pacing rate is
close to URL. Following this, the ACT signal is generated on the
basis of the following equations:
where ACTdr is a drift factor, as discussed more fully below.
Referring to FIG. 5, there is shown the relation between activity
frequency (Fact) and ACT, with ACT presented as a function of
interval. Curve (1) represents ACT for medium activity, while curve
(2) represents ACT for maximum activity. This figure illustrates
how Fact increases as a function of patient activity, and it shows
the conversion from Fact to ACT. It is noted also that in this case
Nact is counted over N intervals, and that ACT increases as a
function of interval (if ACT were determined per a given time
period, each ACT line corresponding to a given activity level would
be horizontal, and not a function of pacing interval). FIG. 5 also
shows the ACT.sub.ref curve (3) superimposed on the ACT curves for
maximum and medium activity. It is noted that the ACT.sub.ref curve
must be at zero at LRL, corresponding to no activity when the
patient is at rest. The curve must extend to the maximal ACT at
URL, corresponding to the highest pacing rate at maximum patient
activity. The ACT.sub.ref relation is chosen to be linear,
according to the following formula:
FIG. 5 indicates, at point 4i, a situation where the measured
activity level at a given interval (I.sub.0) is greater than
ACT.sub.ref, indicating that the pacing rate should be incremented;
and the situation at point 4d, where the activity level is less
than ACT.sub.ref, indicating that the pacing rate should be
decremented. This is summarized as follows:
______________________________________ Point 4i: ACT >
ACT.sub.ref ACT.sub.dif = ACT.sub.ref - ACT < 0 increment pacing
rate Point 4d: ACT < ACT.sub.ref ACT.sub.dif = ACT.sub.ref - ACT
> 0 decrement pacing rate
______________________________________
In the preferred embodiment, as discussed above, there are two
coupled reference curves, as shown in FIG. 5. Thus, at the
programmable LRL, the QT reference curve correlates minimum stress
patient QT at rest and the LRL interval; and the ACT reference
curve correlates the minimum patient activity signal at rest and
LRL. Likewise, at URL, the QT reference curve correlates patient QT
reached at maximum stress and URL, and the ACT reference curve
correlates maximum stress ACT and URL. The curves are coupled in
the sense that each is designed to indicate approximately the same
pacing rate within the LRL-URL range for varying activity or stress
levels.
The decision as to whether to increase or decrease pacing rate
depends upon comparisons of QT.sub.dif and ACT.sub.dif, as
illustrated in FIGS. 7A, 7B-12A, 12B. FIGS. 7A and 7B illustrate
the situation where ACT is greater than ACT.sub.ref and QT is less
than QT.sub.ref. Note that when QT is less than QT.sub.ref, a
higher rate is indicated; when ACT is greater than ACT.sub.ref, a
higher rate is also indicated. Thus, in this situation, both
difference indicators signal an increasing exercise level, calling
for an increase in rate. In FIGS. 8A and 8B, ACT is less than
ACT.sub.ref and QT is greater than QT.sub.ref. Again, both
difference values indicate a decreasing exercise level, so the
algorithm prescribes that the rate goes down. In FIGS. 9A and 9B,
ACT.sub.dif indicates a decremented pacing rate, while QT.sub.dif
indicates an increased pacing rate. In accordance with this
invention, since QT.sub.dif is greater than ACT.sub.dif, the QT
influence prevails and the algorithm calls for the rate to go up.
In FIGS. 10A and 10B, the situation is the same as in that of FIGS.
9A and 9B, except that ACT.sub.dif is greater, and the algorithm
responds to prescribe that the rate goes down. In FIGS. 11A and
11B, ACT.sub.dif signals increasing exercise, while QT.sub.dif
signals decreasing exercise. Since the QT influence is the largest,
the algorithm calls for the rate to go down. In FIG. 12, the
situation is the same as FIG. 11, but since the ACT influence is
the largest, the algorithm prescribes that the rate goes up.
As seen from FIGS. 9A, 9B-12A, 12B, the relative influence of the
QT and ACT signals are compared by the algorithm to determine, in
certain situations, whether the activity parameter or the QT
parameter prevails in causing pacing rate to increase or decrease.
The relative influence of the activity sensor on the pacemaker
response depends upon the comparison between the maximum value of
ACT.sub.ref at URL, and QT for the highest stress level at URL
(QT.sub.STR(max)). The value for QT.sub.STR(max) may not be
precisely known for the patient, but generally can be estimated to
be approximately 30 ms The maximum value of ACT.sub.ref at URL can
be set so as to give any fixed relation to QT.sub.STR(max). The
pacer variable ACTmpl then must be set such that ACTmpl * Nact
matches such maximum ACT.sub.ref at URL. Note that ACTmpl
determines the ACT signal, so if ACTmpl is small, this minimizes
the influence of ACT. Likewise Dact, which represents the linear
variation of ACT.sub.ref with interval, establishes desired
variations of pacing rate with change in activity level, and if
Dact is small, this also minimizes the influence of ACT. On the
other hand, if both Dact and ACTmpl are relatively large, then ACT
has a relatively greater influence on the decision made by the
algorithm to increase or decrease pacing rate.
Referring to FIGS. 13A, 13B, 14A, 14B, and 15A, 15B, there are
represented three situations which illustrate the relative
influence between QT and ACT. In all three figures, ACT indicates
an increasing exercise level, whereas QT indicates a decreasing
exercise level. In the situation of FIG. 13A, 13B, ACTmpl and Dact
are small (A<B), so ACT has little influence (C<D). Since QT
has the greater influence, the rate is caused to go down. In FIG.
14A, 14B ACTmpl and Dact are chosen such that ACT and QT have
substantially equal influence (A=B), and the algorithm will
maintain pacing rate the same (except that each single step it will
increment or decrement). In the situation of FIG. 15A, 15B, ACTmpl
and Dact are relatively large (A>B), causing ACT to have greater
influence (C>D). This results in the algorithm causing the rate
to increase.
Referring now to FIG. 16, there is shown a flow diagram of a
preferred rate algorithm incorporating both the QT and activity
(ACT) variables. The steps of this specific algorithm are carried
out each cycle following delivery of a pacing stimulus. Reference
is also made to FIGS. 7-15 which illustrate the effect of the
algorithm under varying situations.
As indicated at 71, it is first determined whether the pacing rate
is near LRL. This is done by comparing the measured time interval
(Ttx) to the value of the interval corresponding to LRL (T.sub.LRL)
minus 25.6 ms. If Ttx is smaller than this interval, meaning that
it is not within 25.6 ms of lower rate limit interval, then the
program branches to block 72 where it is determined whether the T
wave has been sensed. If yes, or if the actual pacing interval is
within 25.6 ms of T.sub.LRL, the program branches to block 76. If
no T wave has been sensed, such that QT rate information is not
available, the program branches to block 73 where the last measured
QT value is increased by an arbitrary amount, e.g. 50 .mu.s. Thus,
the pacer provides a drift to the QT value which causes it to
increase toward T.sub.LRL when no T waves are sensed. However, when
the pacing rate comes near to LRL, the drift in QT stops. In
practice, the drift of QT is limited to a predetermined value,
since a high activity level sensed by the activity sensor could
maintain the pacer far from LRL, in which case a prolonged period
of absence of T wave sensing would cause the variable QT otherwise
to drift lower than a value corresponding to T.sub.LRL.
Following the decision as to whether to cause QT to drift, the
software makes a decision as to whether to increment or decrement
the rate, in accordance with the steps carried out at blocks 76,
77, 79, 80 and 82. As indicated at block 76, the difference
variables, ACT.sub.dif and QT.sub.dif (as defined above), are
determined by subtracting the ACT and QT values from their
respective corresponding reference values at the current interval,
Ttx. As per the above discussion, an increase in the ACT variable
so that it has a higher value than the ACT.sub.ref variable is an
indication of increase of rate, such that a negative ACT.sub.dif
indicates a desired rate increase, and vice versa. At the same
time, a decrease in sensed QT value to the point that it is less
than QT.sub.ref, causing a positive QT.sub.dif, indicates a rate
increase, and vice versa. Thus, while the difference variables are
comparable in the sense that ACT is treated as a substitute
parameter to QT, the difference in sign needs to be taken into
account. At block 77, the pacemaker determines whether it is in the
QT mode, i.e. whether the QT parameter is being utilized. If no,
the program branches to block 82 where further logic is based on
the activity signal alone. If yes, the program goes to block 79
where the signs (plus or minus) of ACT.sub.dif and QT.sub.dif are
compared. If these signs are found not to be equal, in other words
unequal, the program branches to block 82. In such case, both
variables point to the same direction of change (FIGS. 7 and 8),
and at 82 ACT.sub.dif is utilized to determine whether the rate
should be increased or decreased. If ACT.sub.dif is negative, then
an increase in rate is indicated, and the program branches to block
88. If ACT.sub.dif is not negative, then a decrease in rate is
indicated, and the program branches to block 84. Note, as discussed
above, there can be two situations where the difference values have
equal signs, namely where QT is less than QT.sub.ref and ACT is
greater than ACT.sub.ref ; and where QT is greater than QT.sub.ref
and ACT is less than ACT.sub.ref. However, if at 79, the difference
variables have unequal signs, which happens in four situations, the
program branches to block 80. There it is determined which
parameter has the greatest influence, by determining whether
QT.sub.dif is greater than ACT.sub.dif. If yes, an increase in rate
is called for, and the program branches to block 88. If no, a
decrease in rate is called for, and the program branches to block
84.
At block 84, the current interval Ttx is increased by 5 ms, i.e.
the pacing interval is increased by 5 ms Then, at block 85, new
points on the QT reference curve and ACT reference curve are
calculated, to correspond to the new interval. The new QT.sub.ref
is calculated by increasing QT.sub.ref by an incremental amount
"curve" calculated as follows:
At the same time, a new point on the activity reference curve is
calculated by decreasing ACT.sub.ref with the value Dact.
ACT.sub.ref cannot be decreased below a predetermined minimum.
Thus, the decision to decrease rate results in increasing the
interval by 5 ms, and adjusting the reference points on the
reference curves of both QT.sub.ref and ACT.sub.ref.
If the comparisons made at blocks 79 and 80 indicate an increase in
rate, the program proceeds to block 88 where a calculation of step
size is made. The interval change during increase of rate is made
dependent upon the interval, i.e., Ttx. This feature limits change
of pacing rate at very high rates (corresponding to short
intervals) and permits larger pacing interval changes with
increasing intervals. For the preferred embodiment, the
correspondence between interval and step size is set forth in the
following table:
______________________________________ Interval Stepsize
______________________________________ <614.4 ms 5 ms = 1 * 5 ms
614.4-819 ms 10 ms = 2 * 5 ms 819.2-1024.0 ms 15 ms = 3 * 5 ms
>1024.0 ms 20 ms = 4 * 5 ms
______________________________________
A computer value "LOOP" is set to N=1, 2, 3 or 4 depending upon the
selected step size. Following determination of step size, the
software goes into a loop comprising blocks 92, 94 and 95. At 92, a
new value of QT.sub.ref is determined. QT.sub.ref is decremented by
the amount indicated as "curve", being the same amount set forth
below with respect to block 85. A computer value "temp" is
determined, which is used to increment ACT.sub.ref if URL has not
been reached (as discussed below in connection with block 102).
Temp, which is initialized at zero when the LOOP value is set, is
incremented by the constant Dact. Next, at block 94, the pacing
interval is decremented by 5 ms. The variable LOOP is decremented
by one, and then at 95 it is determined whether LOOP is zero. If
no, the software loops through blocks 92 and 94 again, until LOOP
variable is zero, at which time it exits. Thus, for every step
decrease in interval a corresponding step in QT.sub.ref and
ACT.sub.ref is calculated, to adjust the reference curves. Thus,
QT.sub.ref is decremented and ACT.sub.ref is incremented in the
loop a number of times corresponding to the calculation of step
size at block 88.
Referring now to FIG. 17, there is illustrated a portion of the
algorithm for adjusting operation at URL. The QT.sub.ref curve at
URL is controlled by the factor B, such that QT response at URL can
be adjusted by changing the value of B. The algorithm of this
invention utilizes the logic that if QT shortens more than a
programmable threshold (pacer variable CRVMAX) after pacing rate
reaches URL, then URL was reached too quickly for the patient.
Thus, the condition is met if
The QT.sub.ref curve is adjusted by incrementing B, which
effectively lowers the value of QT.sub.ref at URL. B is incremented
if:
1) the pacing rate has reached URL;
2) after reaching URL, QT shortens additionally by at least an
amount equal to CRVMAX; and
3) QT.sub.ref -ACTdif<QTsave.
Note that if the third condition is not met, then decrementing the
value of B would mean that the pacemaker could not reach a stable
point on the reference curves where QT.sub.ref -QT=ACT.sub.ref
-ACT.
The activity response may also be adjusted as a function of
conditions at URL. Note that ACTmpl * Nact must match the maximum
ACT.sub.ref value at URL. Since the coefficient Dact is programmed
and cannot be automatically changed, ACTmpl is decremented one step
whenever the pacer reaches URL and ACT exceeds the programmable
variable ACTmax. Whenever ACT mpl has been decremented, it will
automatically be incremented once later, e.g., after 8 days.
Still referring to FIG. 17, following an increase of the rate
(decrementing interval), the software goes from block 95 (FIG. 16)
to 97, where it is determined whether pacing rate has reached URL.
Thus, if Ttx is smaller than TURL, upper rate limit has been
reached and the program branches to 105. If no, upper rate limit
has not been reached and the program branches to block 98. At block
98 it is determined whether the pacing rate has come within 25.6 ms
of upper rate limit. If yes, at block 101, a "new URL adapt" is
enabled. Since Ttx is less than T.sub.URL, ACT ref is adjusted by
adding "temp" as calculated at block 92.
Returning to block 105, if URL has just been reached, and new URL
adapt is enabled, the program branches to block 109. At this point
QT is compared to QTsave, as defined above. If QT is less than
QTsave, a software save register is set so that during the next
cycle at 105 the answer is no, and the program exits. In the next
cycle, at 105 the program branches to block 106. If URL has already
been adapted, the program exits; if not, it branches to 112 where
it is determined whether ACT.sub.dif is positive. If yes, the
program branches to block 114. There the difference between
QT.sub.ref and ACT.sub.dif is compared to QTsave. If this
difference is positive, the program exits. If this difference is
less than QTsave, it means that ACT.sub.dif is greater in magnitude
than the increment CRVMAX, such that QT.sub.ref at URL can be
decremented. The program then proceeds to 118 to determine whether
the B variable can be incremented. If it can, at 122 B is
incremented, and then at 124 the new URL adapt flag is disabled.
Returning to block 112, if ACT.sub.dif is not positive, at 113 the
algorithm determines whether the negative of ACT.sub.dif is greater
than the maximum value of ACT. If no, then B can be incremented and
the program branches to 114. If yes, it means that the 5 NACT
counts exceed a predetermined value and the program goes to 117
where it is determined whether automatic decrementing of ACTmpl is
enabled. If yes, at block 121, ACTmpl is decremented, and the
program branches through to 124.
The pacemaker of this invention also provides for automatic
adjustment of rate response at LRL, as set forth in U.S. Pat. No.
4,972,834. When the patient is at rest (and the pacing rate is near
LRL, the pacemaker paces for a while at LRL and calculates an
average QT at LRL. The pacemaker then decreases the pacing interval
by a small amount and calculates a second QT average at the second
interval, near LRL. The difference in two QT averages and the
difference in the two intervals are divided to provide the
coefficient corresponding to pacer variable Dpt. This is compared
to the prior value of the coefficient, and the value of Dpt is then
adjusted one step in the direction of the indicated change. Thus,
if the ratio indicates a greater slope at LRL than had 2.5
previously represented by the value of Dpt, Dpt is increased so
that the QT.sub.ref curve near LRL is steeper. By this technique,
after a number of such slope measurements the QT.sub.ref curve near
LRL is adapted to substantially match the QT curve of the patient's
heart.
Referring now to FIGS. 17, 18A, 18B, 19A, and 19B, there is
illustrated the drift action incorporated in the preferred
embodiment, for adjusting the ACT parameter in situations where the
activity signal does not correlate with QT. It is known that the
activity sensor is not always a proportional indicator, and
excessively high activity level may be indicated in several
situations. For example, if the patient is in rest and yet activity
signals are counted, e.g., caused by respiration or the patient's
heartbeat, the activity indication is too high and should be
decremented. Likewise, there are situations where Nact may be too
high to correspond to the actual exercise level, such as where
vibrations are sensed which are caused by external forces. To
account for these influences, and decrease the ACT signal
accordingly, ACT is periodically compared to ACT.sub.ref, as seen
in block 150. Since the QT.sub.ref curve and the ACT.sub.ref curve
are coupled so that movements along the QT.sub.ref curve should be
matched with movements along the ACT.sub.ref curve, if ACT does not
match ACT.sub.ref, this is an indication that the sensed activity
signal is incorrect. By causing the ACT signal to drift in such a
situation, i.e., by adjusting it to decrease the magnitude of
ACT.sub.dif, the activity information is correlated with the QT
information. To accomplish this, ACT is adjusted by a drift signal,
referred to as ACTdr, according to the following formula:
As can be seen, the drift factor, ACTdr, essentially compensates
for any inaccuracy in the sensed Nact variable. If ACT is measured
to be larger than ACT.sub.ref, ACTdr is increased, thereby causing
ACT to decrease, bringing it back into correlation with QT. At
block 152 of FIG. 17, if ACT.sub.ref is found not to be greater
than ACT, the program branches to block 153, where ACTdr is
incremented by one unit, which results in a decreased value of ACT.
The algorithm checks at block 153 to set an upper limit on the
value of ACTdr. If the comparison at 152 is such that ACT is less
than ACT.sub.ref, then the program branches to block 155, and
decrements ACTdr. A limit is placed on ACTdr such that it cannot go
below zero. In this embodiment drift compensates only for false
positives: a rate indication by the activity sensor which is high
compared with the QT information. Thus, the algorithm periodically
introduces the drift factor compensation into the ACT signal,
thereby either increasing or decreasing the ACT signal. This is
illustrated in FIGS. 18A, 18B, 19A and 19B for the situation where
ACT is greater than ACT.sub.ref. As indicated in FIG. 18B, the QT
signal is greater than QT.sub.ref, indicating that the pacing rate
should go down, i.e., interval should increase. However, ACT is
greater than ACT.sub.ref, and ACTdr is incremented. When the
pacemaker stabilizes to a final situation, as shown in FIGS. 19A,
19B, ACTdr has been incremented so that the actual ACT has come to
equal ACT.sub.ref. In FIGS. 19A, 19B, for the increased pacing
interval, the ACT curve with drift is shown at curve 6, being
displaced downwardly from curve 5 without drift (which corresponds
to actual Nact). Note also that the position on the QT.sub.ref
curve has changed so that it corresponds to a lower actual stress,
and QT equals QT.sub.ref. In the reverse situation where ACT is
less than ACT.sub.ref, ACTdr would be decremented to bring the ACT
signal back into correlation with the QT signal, limited by the
fact that only positive ACT.sub.dr values are allowed.
The embodiment as illustrated permits drift to correct only for a
false positive, i.e., the situation where the second sensor value
(ACT) gives too high an indication compared to the first sensor
value (QT). However, the algorithm can be adapted to apply drift
for a false negative, e.g., by decrementing ACT.sub.dr to a
negative value. Also, it is to be understood that the drift feature
may be programmed to compensate the fast sensor by other than fixed
steps in order to reduce the fast sensor influence to or toward
zero over time.
As discussed above, the system and method of this invention is
applicable to employing two or more control parameters. It is noted
that hardware simplification of the system can be achieved if a
second or extra control parameter can be obtained without the need
of an extra sensor means. This can be achieved in a system where QT
is a primary parameter, by utilizing another parameter obtained by
the sensed heartbeat signal. For example, the amplitude of the T
wave (TwA) can be utilized as a control parameter separate from QT,
as can other features of the Q or T wave portions of the sensed
heartbeat signal. See U.S. Pat. No. 4,305,396. The parameter TwA
has a reasonably quick response, i.e., reacts to changing patient
conditions more quickly than does the QT interval. Thus, TwA is a
good choice of a second parameter to be used in combination with
QT. In accordance with this invention, the amplitude of the T wave
can be determined each cycle by signal processing circuitry such as
illustrated at block 58 of FIG. 1. Data for a TwA reference curve
may be programmed into the pacing system to provide a coupled
reference curve, as well as conversion data for converting the
amplitude signal into a comparable signal with units of ms. Such a
system has the advantage of needing no extra sensor, since both
control parameters are obtained from the same pacing lead 53 as is
conventionally used for introducing stimulus pulses into the
ventricle. Alternately, the TwA or any other control parameter
derived from the sensed heartbeat signal may be utilized as a third
control parameter. Of course, while the preferred embodiment of
this invention has been illustrated as using QT as a primary
control parameter, it is to be understood that any other sensor
signal could be the primary control parameter, and the secondary
parameter would be converted into the units of the first parameter.
This invention embraces control parameters such as AR interval, R
wave morphology, T wave morphology, impedance changes, and other
variables including those set out above under the Description of
the Background, in any combination of two or more.
While the invention has been illustrated by description of
preferred embodiments, it is noted that it is limited only by the
claims hereto. For example, the important feature of providing
comparable parameter variables may be utilized without some of the
other techniques as disclosed. For example, instead of obtaining
difference values (e.g., QT.sub.dif and ACT.sub.dif), the parameter
values may be compared on another basis. Further, actual rate
control may be achieved by changing pacing interval directly to the
point on the reference, or correlation curve corresponding to the
sensed parameter, rather than simply incrementing or decrementing
by a given amount. Such variations are programmable and are within
the scope of the invention.
* * * * *